The present invention relates to an improved process for preparing pyridine compounds substituted with fluorine.
Pyridine compounds possessing fluoro substituents are useful intermediates in the preparation of biologically active compounds, including herbicides, plant growth regulators, insecticides, and fungicides. The compound 2,3-difluoro-5-(trifluoromethyl)pyridine, for example, is useful in the preparation of the herbicide 2-(4-((3-fluoro-5-(trifluoromethyl)-2-pyridinyl)oxy)phenoxy)propanoic acid. In a like manner, the compound 3,5-dichloro-2,4,6-trifluoropyridine is useful in the preparation of the herbicide 4-amino-3,5-dichloro-6-fluoro-2-pyridinyloxyacetic acid.
Current processes for the preparation of pyridine compounds possessing fluoro substituents, while functional, often produce large amounts of waste products, are inefficient, or are otherwise costly. This is particularly true with respect to 3- and 5-fluoropyridine compounds. One of the simpler methods known, which involves the replacement of chloro substituents with fluoro substituents by treating with hydrogen fluoride at an elevated temperature in the presence of an activated charcoal catalyst, is disclosed in U.S. Pat. No. 4,563,530. The process described therein, however, suffers from a relatively slow reaction rate and from a relatively rapid rate of deactivation of the catalysts employed.